1. Field of the Invention
The present invention relates to a scaling method, involving controlling the magnification or minification of reproduction, applied to scanning exposure-reproduction of an original by a predetermined factor to produce a printing film, and also to a reproduction apparatus, according to the principles of scaling scan.
2. Description of the Background Art
An example of such scanning exposure-reproduction apparatus is disclosed in Japanese Patent Laying-Open Gazette No.2-137837 by the applicant of the present invention.
A scanning exposure-reproduction apparatus shown in FIG. 1 focuses a scaled reproduced image of an original held on a holder 2 onto a cylindrical drum 10 by means of scanning exposure. The scanning exposure-reproduction apparatus includes: the holder 2 horizontally movable along a pathway 1; the cylindrical drum 10 for holding a sensitized material, which is synchronously rotatable with the movement of the holder 2 and functions as a projection base; a series of optical devices for scaling projection including a lens 15 and mirrors 14b and 14c, which are disposed around the cylindrical drum 10; a series of optical devices for photographing including a light source 13 and a mirror 14a, which are disposed on the upper face of the holder 2; sensitized material conveying units 17 and 18 for feeding a sensitized material 19 in and out of the cylindrical drum 10; and a control panel 21 for setting a reproduction scale and other conditions.
The holder 2 and the optical devices for photographing constitute a photographing optical system, which is used for photographing a slit image spanning the width of an original and relatively moving the photographing position of the slit image in the longitudinal direction of the original. The speed of the relative movement is changeable within a predetermined range.
The cylindrical drum 10 and the optical devices for scaling projection, on the other hand, constitute a projecting optical system, which projects the slit image along the width of the sensitized material and relatively moves the exposure position of the slit image in the longitudinal direction of the sensitized material at a predetermined speed.
In scaling scan, the cylindrical drum 10 actuated by a driving motor (not shown in figures) rotates at a fixed speed while the holder 2 actuated by a holder driving motor 6 moves at a predetermined speed based on a reproduction scale input through the control panel 21. The reproduction scale in the longitudinal direction of the sensitized material represents a ratio of the rotating speed of the cylindrical drum 10 to the moving speed of the holder 2. The moving speed of the holder 2 is thus determined with both the rotating speed of the cylindrical drum 10 and the input value of the reproduction scale. The reproduction scale in the transverse direction of the sensitized material is determined by the position of the lens 15 and the mirrors 14b, 14c along the optical axis.
The scanning-exposure reproduction device allows free setting of the reproduction scale in the longitudinal direction based on the moving speed of the photographing optical system and thus realizes scaling reproduction in spite of its limited space.
With recent extensive use of printed matter, reproduction devices of high quality and of a wider scaling range are highly required for producing a desirable printing plate from an original. For example, an original of A2 size should be enlarged to printed matter of B0 or A0 sizes.
Since the reproduction scale in the longitudinal direction of the sensitized material represents the ratio of the rotating speed of the cylindrical drum 10 to the moving speed of the holder 2 as mentioned above, a specific motor having a sufficiently wide range of rated speed change is used as the driving source of the holder 2 so as to expand the range of reproduction scale.
Such a driving motor with a wide range of rated speed change, however, has a complicated structure and thus requires time- and labor-consuming maintenance and high manufacturing cost.
On the other hand, enlargement by a large scale factor without the specific driving motor requires the steps of: enlarging an original by a predetermined factor to produce printed matter and further enlarging the printed matter by a certain factor. This method uses the intermediate printed matter as the original of final reproduction and thus causes poor image quality of the final printed matter. Similar problems arise when an original is reproduced by a reducing ratio.
With recent requirements for better image quality, unevenness of an image by a reproduction device, which has been neglected, needs to be removed. In the scanning exposure-reproduction apparatus, slight changes of the moving speed causes unstable exposure of an original and produces uneven stripes on a recording image and thereby lowers the quality of the image.
The photographing optical system including the holder 2 (hereinafter referred to as the holder system) and the projecting optical system including the cylindrical drum 10 (hereinafter referred to as the projection system) are driving systems or vibration systems each having a vibration source. When the frequency of a driving motor or a reduction gear functioning as the vibration source is comparable to the natural frequency of the driving system, resonance causes change of the moving speed of the holder 2 or the cylindrical drum 10, which results in poor image quality. Accordingly, actuation of the holder system in the vicinity of the resonance point is not recommendable.
In a reproduction device which executes reproduction by a predetermined factor m within a fixed scale range of a driving source (driving motor), the speed of rotation of the driving motor is previously set not to be close to the resonance point. In some cases, the speed of rotation previously set, however, exceeds the range of the rated driving speed or the rated rotating speed of the driving motor corresponding to the scale range; that is, actuation in the vicinity of the resonance point can not always be avoided.
Use of a specific motor with a sufficiently wide range of rated speed change or use of a highly rigid driving system may be considered to keep away from or shift the resonance point. One driving system generally has plural resonance points, and thus these methods are neither practical nor economical.